Implement recipient stream indexing

[Jagadeeshftw]

Description
Add recipient-based stream index for recipient portal and withdraw workflows.

Requirements and context

• Must be secure, tested, and documented.
• Must stay consistent if recipient updates are supported.

Suggested execution

• Branch: git checkout -b feature/recipient-stream-index
• Add recipient index data structure and update hooks.
• Add tests for lifecycle consistency and retrieval.
• Add docs with expected sort order.

Example commit message
feat: implement recipient stream index

Guidelines

• Minimum 95 percent test coverage.
• Clear documentation.
• Timeframe: 120 hours.

[Emmanuelluxury]

I’d like to work on this issue because it sits at the intersection of system design, financial logic, and secure contract architecture — areas I actively specialize in and enjoy building within. The problem requires careful handling of state transitions, precision math, and security constraints, which makes it both technically meaningful and impactful to the overall reliability of the protocol.

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[Fayvor22]

@Fayvor22 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I would love to handle this and get it resolved, please assign

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[YahKazo]

@YahKazo has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I would love to work on this

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[Samaro1]

@Samaro1 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

gm gm boss, I would love to work on this issue, I will provide a satisfactory and efficient PR in 4 hours tops

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[Pvsaint]

@Pvsaint has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I'm a software dev, may I take on this issue if it's still available?
ETA 2hrs

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